The present invention relates to an optical information recording medium in which information such as a video signal, an audio signal and a software of a computer is recorded and a reading system for reading the information recorded on the optical information recording medium.
Recently, a software or contents of a computer are recorded on a compact disc as an optical information recording medium, and the compact disc is distributed to a user. In the compact disc, the information is recorded on a recording surface thereof in the form of grooves and lands. The grooves and lands are coated with a reflecting film made of aluminum by vacuum deposition. Thus, a reflecting surface is provided on the recording surface having the pits. In order to read the recorded information of the disc, a reading system emits a light beam which is projected on the pit of the recording surface and the light reflected from the reflecting surface of the recording surface is detected. Thus, the information is read out.
Since the quantity of information to be distributed at one time has been increased, it is desirable to increase the recording capacity of the disc. Therefore, it is considered to form the pits on the disc more finely. On the other hand, it is considered the reading system is employed with a light source having a short wavelength, or an objective having a large numerical aperture, thereby improving a resolving power of a pickup.
However, there is a limit to increase the recording capacity of the disc. Furthermore, there is a problem that if the recording capacity is increased, it takes a longer time than a conventional disc to access the necessary information.
In order to eliminate such inconvenience, an optical disc having two recording surfaces is known.
FIG. 10 shows an optical disc having two recording surfaces. An optical disc 1 comprises a substrate 2 made of polycarbonate, a recording layer 4 provided on the substrate 2, and a protection layer 3 made of acrylic resin and provided on the recording layer 4 for protecting the recording layer.
The recording layer 4 is made of ultraviolet curing transparent resin and has a first recording surface 7 formed on an upper portion thereof (as viewed in FIG. 10) and a second recording surface 8 formed on a lower portion thereof (as viewed in FIG. 10). On the first and second recording surfaces 7 and 8, information is recorded in the form of grooves and lands. A half mirror film 5 is coated on the first recording surface 7. The half mirror film 5 is made of gold so as to reflect 40% of light beams applied to the first recording surface 7 from a reading system (not shown). A reflecting film 6 is coated on the second recording surface 8. The reflecting film 6 is made of aluminum so as to reflect all of the light beams applied to the second recording surface 8.
In order to read information recorded on the first recording surface 7 of the disc 1, a light beam emitted from the reading system is focused on the pit of the first recording surface 7 and reflected beams of 40% are read out. On the other hand, in order to read information recorded on the second recording surface 8, the light beam emitted from the reading system is focused on the pit of the second recording surface 8 and reflected beams of 36% (=60%.times.60%) are read out.
Namely, in such a structure, if a focal point is properly adjusted on the pit of either of the recording surfaces 7 and 8, the information on the selected recording surface can be read out. Consequently, the recording capacity of the optical disc can be increased twice that of the disc having the single recording surface.
However, since the half mirror film 5 is provided in the disc 1, it affects the quantity of light to be reduced so that the signal-to-noise ratio is deteriorated. Furthermore, the light beams which do not contribute the reading information are detected by the reading system as stray light which affects a servo signal. If the number of recording surfaces is increased, for example, more than three, the quantity of light is further reduced to increase the stray light. Therefore, only two recording surfaces are provided.
In order to solve the aforementioned problems, there has been proposed a reading system employed with a principle of Optical Coherence Domain Reflectometry (OCDR) reported on page 899 of No. 12 of "Optics letters Vol. 21".
FIG. 11 shows a reading system employed with the principle of OCDR. A reading system 10 is provided for reading information recorded on a multi-layered optical disc 9. The reading system 10 comprises a light source 11, a lens 17, a beam splitter 12, a lens 18, an optical phase shifter 13, a reflecting mirror 14, an objective 16 and a photodetector 15.
As the light source 11, a super luminescent diode (SLD) having a temporal incoherent characteristic is employed. At the the beam splitter 12, a light beam emitted from the light source 11 is divided into reference light and detecting light for interfering with each other. The information recorded on the disc 9 is read out in accordance with the interference. The reflecting mirror 14 is provided to be served as a reference surface and moved in the directions shown by arrows.
In the light source 11 having a temporal incoherent characteristic, interference between light waves generates. However, the interference generates only when an optical path difference is approximately zero. To this end, the mirror 14 is moved in order that the distance between the beam splitter 12 and the mirror 14 and the distance between the beam splitter 12 and one of the recording surface of the disc 9 becomes equal to each other.
Referring to FIG. 12, the optical disc 9 comprises an upper substrate 2A made of polycarbonate, a recording layer 4A provided on the upper substrate 2A, and a lower substrate 3A made of acrylic resin and provided on the recording layer 4A for protecting the recording layer. The recording layer 4A is made of ultraviolet curing transparent resin and has upper and lower recording surfaces 19 and 20. Information is recorded on the respective recording surfaces 19 and 20 in the form of grooves and lands.
Furthermore, the upper substrate 2A and the recording layer 4A are different in refractive index, so that a part of quantity of the incident light reflects from the upper recording surface 19. In addition, the depth of the groove is determined such that a phase difference between the phase of the incident light and the phase of the reflected light becomes .pi., so that the reflected light interferes with the incident light, thereby largely reducing the reflected light from the groove.
At the beam splitter 12, the reference light is applied to the reference mirror 14 through the optical phase shifter 13. The reference light reflected back to the beam splitter 12 is applied to the photodetector 15. On the other hand, the detecting light is applied to one of the recording surface of the disc 9 and reflected back to the beam splitter 12 and applied to the photodetector 15 through the objective 16. Thus, the reference light and the detecting light are interfered with each other, thereby enabling the reading of the information on the selected recording surface of the disc.
Since a part of the incident light is reflected from the upper recording surface 19, it is not necessary to provide the half mirror film 5 formed in the disc 1 of FIG. 10. Therefore, the disc having a multi-layered recording surfaces can be easily obtained. Thus, it is possible to increase the recording capacity of the disc. In the reading system, the quantity of light is prevented from reducing, and the information can be read out with accuracy at a high speed.
However, in such a disc and a reading system, it is necessary to make the distance between the beam splitter 12 and the mirror 14 and the distance between the beam splitter 12 and the recording surface of the disc 9 to be equal in order to cause both light waves to interfere with each other. Therefore, the lateral vibration and change of the position of the disc during the rotation of the disc 9 which are caused by the deformation of the disc must be accurately controlled. Accordingly, process and cost for manufacturing the disc 9 and the reading system 10 are increased.